A nitride semiconductor such as gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN), or a material composed of a mixed crystal of GaN, AlN and InN generally has a wide band gap. These materials are utilized as a high-power electronic device, a short-wavelength light emitting device or the like. Among these, a technology associated with a field-effect transistor (FET), in particular, a high electron mobility transistor (HEMT) has been developed as a high-power device (e.g., Patent Document 1). A high electron mobility transistor (HEMT) including a nitride semiconductor may be utilized for a high-power and high-efficiency amplifier, a high-power switching device, and the like.
The HEMT having a nitride semiconductor generally includes an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure formed on a substrate, in which a GaN layer serves as an electron transit layer. Note that the substrate may be formed of sapphire, silicon carbide (SiC), gallium nitride (GaN), silicon (Si), and the like.
The GaN, which is a kind of the nitride semiconductor, includes a high saturation electron velocity or a wide band gap. Hence, the GaN may be able to acquire superior pressure resistance and exhibit excellent electric characteristics. Further, the GaN is polarized in a [0 0 0 1] direction parallel to a c-axis (wurtzite form). Accordingly, when the AlGaN/GaN heterostructure is formed, piezoelectric polarization may be induced by lattice strain due to the difference in the lattice constant between the AlGaN and GaN. As a result, highly-concentrated two-dimensional electron gas (2DEG) may be generated near an interface of the GaN layer.
More specifically, an example of a related art HEMT is described with reference to FIG. 1. The HEMT having a structure of FIG. 1 includes an (AlN) buffer layer 912, an i-AlGaN buffer layer 913, an electron transit layer 914, and an electron donation layer 915 formed on a substrate 911. The HEMT further includes a gate electrode 921, a source electrode 922 and a drain electrode 923 on the electron donation layer 915. With such a structure, 2DEG 914a may be generated in the electron transit layer 914 near the interface between the i-GaN forming the electron transit layer 914 and the i-AlGaN forming the electron donation layer 915.
From the standpoint of safety, it is preferable that power devices such as the high-power and high-efficiency amplifier or the high-power switching device have normally-off characteristics capable of shutting off electric current flow when the bias is at a zero voltage. However, in the related art example illustrated in FIG. 1, the 2DEG generated at the interface includes high concentration of electrons. Hence, a negative bias may need to be applied to the gate electrode 921 for shutting of the electric current flow. Accordingly, the device utilizing the GaN may achieve a low on-resistance exceeding the limitation of a device utilizing Si. However, it may be difficult for the device utilizing the GaN to be applied to an inverter as a power device.
Hence, in order for the HEMT having the AlGaN/GaN heterostructure to be in a normally-off status, it may be necessary to shift a threshold voltage to a positive side by decreasing electrons of the 2DEG 914a directly beneath the gate electrode 921. The threshold voltage may be shifted to the positive side by controlling the generation of the 2DEG electrons based on the polarization effect obtained by thinning the AlGaN film or decreasing Al composition. However, with the above methods, on-resistance may be increased with the decrease of the 2DEG electrons.
Further, as a method for causing the HEMT having the AlGaN/GaN heterostructure to be in a normally-off status, there is disclosed the HEMT having the AlGaN/GaN heterostructure having a p-GaN layer layered directly beneath the gate electrode (e.g., Patent Document 2). In this structure, the concentration of the 2DEG electrons in the electron transit layer may be decreased due to the holes injected from the p-GaN layer directly beneath the gate electrode. Accordingly, the threshold voltage may be shifted to the positive side to cause the HEMT having the AlGaN/GaN heterostructure to be in a normally-off status without increasing the on-resistance.